Crack Toughness Measurements of High-Strength Steels

The influence on fracture toughness of relative specimen thickness in the center-precracked tension specimen and crack length in the precracked bend specimen was evaluated. The results indicated that the nature of the crack growth curve in the precracked sheet tension test could be related directly to the fracture mode transition temperature. Tests conducted below the fracture mode transition region provided only catastrophic crack extension, while testing within the transition region resulted in measurable slow growth which was initiated by a sharp discontinuity (pop-in). Above the brittle-to-ductile transition slow crack growth was observed, but no meaningful fracture toughness could be measured.

The study of the influence of crack length on the fracture toughness obtained in the notch bend test indicated that valid values of KIc were obtained when the crack size (a) was 2.5 times the (K/σy)2 ratio in accordance with ASTM recommendations. However, it appears that the crack size requirements could be reduced by a factor of 2.5 without producing variations of greater than 5 percent in the measured fracture toughness value provided that the thickness requirement was satisfied.

The available KIc values for a variety of high-strength steel types were also presented graphically as a function of both strength level and test temperature.